As shown in FIG. 29, in a switching amplifier shown in FIG. 7 of PTL 1, a response peak occurs at a high frequency around the cutoff frequency of a low-pass filter at the time of a high impedance load or no load. In order to suppress this, as shown in FIG. 30, providing a damper formed by a capacitor CD and a resistor RD in parallel to a low-pass filter capacitor C may be considered.
In addition, in a case where a MOSFET is used as switching elements FET1 and FET2, reverse recovery where a turned-on state transitions to a turned-off state is not instantly performed due to an embedded diode (body diode) of the MOSFET. As a result, a reverse current flows through the embedded diode due to the carrier accumulation effect even if the switching element body is turned off, and thereby a through current flows from a turned-on switching element to a turned-off switching element. In order to prevent the through current, it is considered that, as shown in FIG. 31, high-speed diodes D3 and D4 for preventing back-flow are provided in series to a switching element, and high-speed diodes D5 and D6 for bypass of a counter electromotive force are provided in parallel to the switching element.
However, power loss occurs in the above-described damper, and thus power loss also occurs in the high-speed diodes D3 and D4 for preventing back-flow. As a result, there is a problem in that efficiency is lowered. In order to solve the problem, a digital power amplifier shown in FIG. 1 of PTL 1 has a configuration shown in FIG. 32.
In the digital power amplifier, as a feedback circuit from a connection point between a coil L and a capacitor C forming a low-pass filter to an analog amplifier OP, a serial circuit of a capacitor Cf and a resistor R2f is configured. By applying the serial circuit as a feedback circuit, a damping effect is increased by a multiple of a loop gain, and thus high resistance can be used for a damping resistor. For example, a resistor of several tens of kΩ is applied as the resistor R2f, and a capacitor of 100 pF is applied as the capacitor Cf. As such, the serial circuit also has the function of the damper (the capacitor CD and the resistor RD) shown in FIG. 30. In addition, by applying the serial circuit as a feedback circuit, a phase delay of the high frequency around the cutoff frequency of the low-pass filter is suppressed to 90 degrees, and thus a phase delay which reaches at a maximum of 180 degrees in the low-pass filter is suppressed to 90 degrees, thereby suppressing oscillation. With this serial circuit, power loss can be considerably reduced as compared with the circuit in FIG. 30.
In addition, in a digital amplifier block 10 of the digital power amplifier, as shown in FIG. 33, a switching element SW1, a coil L11, a coil L12, and a switching element SW2 are connected in this order between the positive and negative power supply lines +B and −B. In addition, the connection point between the switching element SW1 and the coil L11 is connected to a cathode of a high-speed diode D12, and an anode of the high-speed diode D12 is connected to the negative power supply line −B. In addition, the connection point between the coil L12 and the switching element SW2 is connected to an anode of a high speed diode D11, and a cathode of the high-speed diode D11 is connected to the positive power supply line +B. In addition, the connection point between the coils L11 and L12 is connected to one end of a low-pass filter coil L.
As shown in FIG. 33, in a state where only the switching element SW1 is turned on, a power supply current I11 flows, and thus energy is accumulated in the coil L11 as well as the low-pass filter coil L. Here, if the switching element SW1 is turned off (the turned-off state of the switching element SW2 is maintained) through dead time control when a switching element in a turned-on state is turned off, a current I12 shown in FIG. 34 flows due to counter electromotive forces of the low-pass filter coil L and the coil L11. In other words, the coil L11 makes a current flow due to the counter electromotive force thereof and thus draws a current caused by the counter electromotive force of the low-pass filter coil L into the coil L11. As such, the coil L11 prevents the current caused by the counter electromotive force of the low-pass filter coil L from flowing through the switching element SW2 side (an embedded diode side of the switching element SW2). That is to say, the coil L11 shows a back-flow preventing function in the same manner as the back-flow preventing diode D4 shown in FIG. 29.
As such, the coil L11 shows a back-flow preventing function in the same manner as the back-flow preventing diode D4 shown in FIG. 29. As described above, power loss occurs in the back-flow preventing diode D4 but power loss does not occur in the coil L11.